A gas turbine engine includes a turbine section fluidly connected to a combustor by a primary flow path. The turbine section includes a first portion at a high pressure relative to a second portion. A thermally isolated cooling plenum is positioned radially inward of the primary flow path. The cooling plenum is defined by a forward wall, a shaft structure, an aft wall, and an inner diameter wall of the primary flow path. Air in the thermally isolated cooling plenum is thermally isolated from air in the primary flow path. At least one cooling air system is operable to provide cooling air to the thermally isolated cooling plenum.